Nuclear reactors have their fuel contained in sealed cladding for the isolation of the nuclear fuel from the moderator/coolant system. The term cladding, as used herein, refers to a zirconium based alloy tube. Often the cladding will be composed of various layers including a zirconium alloy substrate and an unalloyed zirconium barrier.
The cladding--nominally in the order of 0.030 inches thick--is formed in the shape of a tube with the nuclear fuel contained typically in pellet form therein. These pellets are stacked in contact with one another for almost the entire length of each cladding tube, which cladding tube is in the order of 160 inches in length. Typically, the cladding tube is provided with springs for maintaining the axial position of the fuel pellets and some designs include so-called "getters" for absorbing hydrogen. The internal portions of the fuel rod are pressurized with helium to help conduct the heat from the fuel material to the cladding.
Zirconium and its alloys, under normal circumstances, are excellent for nuclear fuel cladding since they have low neutron absorption cross sections and, at temperatures below about 350.degree. C., are strong, ductile, extremely stable and relatively nonreactive in the presence of demineralized water or stem. "Zircaloys" are a family of corrosion-resistant zirconium alloy cladding materials. They are composed of 98-99% by weight zirconium, with the balance being tin, iron, chromium, and nickel. "Zircaloy-2" and "Zircaloy-4" are two widely-used zirconium-based alloys for cladding.
Zircaloy cladding defects may occur due to various causes including debris induced fretting and pellet-cladding interaction. In the first of these, debris lodges next to the cladding and vibrates or frets against the cladding wall under the influence of the passing steam/water mixture. Such vibration continues until the cladding wall is penetrated. Pellet-cladding interaction is caused by the interactions between the nuclear fuel, the cladding, and the fission products produced during the nuclear reaction. It has been found that this undesirable effect is due to localized mechanical stresses on the fuel cladding resulting from differential expansion and friction between the fuel and the cladding in coincidence with corrosive fission product species causing stress corrosion cracking.
To combat defects due to pellet-cladding interaction, some cladding includes pure zirconium barrier layers metallurgically bonded to the inner surface of the tubing. The pioneering work on bamer layer cladding is described in U.S. Pat. Nos. 4,200,492 and 4,372,817 to Armijo and Coffin, U.S. Pat. No. 4,610,842 to Vannesjo, and U.S. Pat. No. 4,894,203 to Adamson, each of which is incorporated herein by reference for all purposes. Barrier layers have been found to effectively prevent damage to the cladding due to interaction with the pellet. However, if the cladding wall is compromised in some manner (e.g. perforated by debris fretting), and water enters the fuel rod interior, the protection afforded by the barrier layer can be reduced. This is because the steam produced by water within the fuel rod can oxidize the barrier layer.
To protect the zirconium barrier from oxidation should a cladding breach occur, a three layer structure may be employed. See e.g., U.S. patent application Ser. No. 08/091,672 entitled METHOD FOR MAKING FUEL CLADDING HAVING ZIRCONIUM BARRIER LAYERS AND INNER LINERS and U.S. Pat. 5,341,407 issued Aug. 23, 1994. entitled INNER LINERS FOR FUEL CLADDING HAVING ZIRCONIUM BARRIER LAYERS, both of which were filed on Jul. 14, 1993, assigned to the assignee hereof, and incorporated herein by reference for all purposes. Such structures include a corrosion resistant inner liner bonded to the fuel side of the barrier. Unfortunately such linings may still be susceptible to a condition known as localized hydriding which can lead to secondary defects in the cladding.
After a fuel element has suffered a primary breach, it can still be used for some period of time in a reactor. However, if a "secondary failure" occurs as a result of coolant entering through the primary breach, the fuel element may have to be taken out of service. It has been observed that secondary failures are often much worse than the primary failures. Post-mortem studies of fuel rods indicate that the secondary failures are often due to localized hydriding of the cladding.
The primary breach in the cladding walls leads to ingress of water which instantly flashes to form steam. Some of the steam most likely reacts with the zirconium on the inner surface of the cladding to give off hydrogen. This is the hydrogen which leads to the formation of the secondary hydride failures.
Normally, zirconium metal is covered with a thin protective layer of zirconium oxide which protects against hydride formation. If this protective zirconium oxide layer is compromised (as by, for example, a scratch), the oxygen and/or steam in the surrounding atmosphere tends to immediately regenerate the protective zirconium oxide over the compromised area. If, for some reason, the zirconium oxide protective layer is not regenerated, the compromised site may support formation of a hydride. If this occurs, a protective zirconium oxide layer can no longer be formed over the hydride site. Thus, the hydride will continue to grow and damage the cladding as long as sufficient quantities of hydrogen gas are present in the environment.
In the environment within a fuel cladding after a primary breach, a condition known as "oxygen starvation" sometimes results. This condition is most often brought about when the ratio of hydrogen to steam increases beyond an "oxygen starvation" level. At this point, there is insufficient oxygen in the environment to regenerate the zirconium oxide protective layer when it becomes defective or when a bare zirconium region is exposed. Thereafter, the hydrogen will permeate the defective oxide or react at the bare zirconium site to generate hydrides. The brittle zirconium hydride takes up more volume than the pure zirconium. This leads to formations on the cladding variously characterized as hydride "bulges", "blisters", "sunbursts".
It follows that if a high ratio of steam to hydrogen in the cladding interior could be maintained along the axial length of the fuel rod (i.e., if oxygen starvation conditions could be prevented or delayed), secondary hydride damage and the resulting fuel degradation could be ameliorated. Thus, there exists a need for cladding that has the benefits of bilayer cladding while resisting formation of hydride defects in the event of a primary cladding breach.